1. Field of the Invention
This invention relates generally to secure communications, and more specifically to a method for rendering a teletypewriter using a mosaic printer (i.e., a printer operating on the principle of a dot matrix, for example a 5.times.7 matrix of dots) whereby the dots are caused by needles driven at high velocity by electromechanical drives. The electromechanical drives place a high impulsive load on a power supply causing superimposed modulation thereon and on associated power utility lines to the power supply whereby the superimposed modulation can be reconstructed to determine the registration and symbols of the printed text. Similarly, the high currents in the electromechanical drives and connecting lines tend to radiate into the surrounding space such that a receiver for electromagnetic radiation pulses some distance away can be used to covertly deduce the registration and symbols of the printed text. Simple inductive pickup loops placed on the power utility lines can similarly be used some distance away from the teletypewriter printer to receive drive pulse information to deduce the printed text.
2. Description of the Prior Art
The prior art shows mosaic printers with an integer number of printing needles driven by separate electromechanical drives connected to a selection logic for selecting the needles to be actuated, having an electrical power source loadable impulsively. Mosaic or "needle" printers have been on the market for some time and are generally well known. They are fast and relatively quiet. The printing needles are individually controlled from a selection logic. Generally the needle drives are arranged perpendicularly to the writing direction which results in printed dot columns. Each complete symbol is made up of several adjacent columns forming a dot matrix.
Higher and higher printing speeds are being aimed at with advanced technology. As a result, the needles have to be accelerated and stopped very rapidly. In the prior art the most common drive is the electromechanical drive consisting of a solenoid exerting a repelling force on a needle movable along the axis of the solenoid. The existence of a high impulsive field in the solenoid is the reason why mosaic printers are not used whenever the text is likely to contain confidential information.
The pertinent classes and subclasses; i.e., class 101 "Printers and Printing", Class 178, subclasses 23, 37, and 99 "Telegraphy" as well as Class 400, subclass 89 "Cryptography" related to typewriting, do not show any means for falsifying the electromagnetic pulse radiation of an electromechanically driven printer to eliminate correspondence thereof to the printed symbols.